1. Field
One or more aspects of embodiments of the present invention relate to lithium secondary batteries.
2. Description of the Related Art
With the development of compact and mobile electronic devices such as digital cameras, cell phones, laptops, and personal computers, there has been an increasing demand for lithium secondary batteries as an energy source for these devices. Also, the spread of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs), has prompted the development of high-capacity and safe lithium secondary batteries.
In connection with the development of lithium secondary batteries, research on various methods of increasing battery capacity has been conducted.
According to one of the methods of increasing battery capacity, a thick-film electrode having a high current density and a thick electrode material mixture layer may be used in lithium secondary batteries. However, although capacity and energy density per unit volume of a battery may increase according to this method, various other properties of the battery may deteriorate. Particularly, as thickness of the electrode increases, impregnation properties of an electrolytic solution and the mobility of lithium ions deteriorate. In addition, as the thickness of the electrode increases, electrical resistance applied to an electrode plate or resistance of lithium ions increases, thereby reducing rate properties. Accordingly, lifespan characteristics, characteristics at low temperature, and the like may deteriorate.
These properties may deteriorate more in thick-film negative electrodes rather than in thick-film positive electrodes. This is because in typical lithium batteries, electrolytic solutions are polar aprotic organic solvents, negative active materials are hydrophobic carbonaceous materials, and positive active materials are hydrophilic lithium metal oxides. Thus, in a thick-film negative electrode including a large amount of carbonaceous material, impregnation properties of the electrolytic solution and the mobility of lithium ions deteriorate more than in a thick-film positive electrode, and as a result, lithium is deposited on the surface of the negative electrode and forms lithium dendrite. Furthermore, continuous side reactions between the lithium dendrite and the electrolytic solution may increase resistance, and decrease in the amount of the electrolytic solution may further deteriorate other properties of the battery.
Therefore, there is a need to develop methods of improving rate properties and lifespan characteristics of lithium secondary batteries that include a thick-film negative electrode by improving the impregnation of the electrolytic solution into the thick-film negative electrode and the mobility of the lithium ions.